#include <bits/stdc++.h>
#define REP(i, a, n) for(ll i = ((ll) a); i < ((ll) n); i++)
using namespace std;
typedef long long ll;

string S;

int main(void) {
  cin >> S;

  map<char, ll> mp;
  REP(i, 0, 13) mp[S[i]]++;

  char ans = '*';
  for(auto p : mp) {
    char c = p.first;
    ll cnt = p.second;
    if(cnt > 2) {
      cout << "Impossible" << endl;
      return 0;
    }
    if(cnt == 1) {
      if(ans != '*') {
        cout << "Impossible" << endl;
        return 0;
      }
      ans = c;
    }
  }
  cout << ans << endl;
}